Mating reactive cells of paramecia attach to polystyrene petri dishes (Falcon 1007) at the tips of ventrally located cilia. This attachment also initiates the first steps of nuclear activation (micronuclear migration) which normally occurs during the conjugation process. Attachment is induced by certain hydrophobic reagents and by immune IgG which has been directed against the surface localized i-antigen or by immune IgG which blocks mating in reactive cells. Treatment of mating reactive cells with IgG purified from antiserum that blocks the mating reaction greatly enhanced the attachment to polystyrene even though the cells had lost mating reactivity. These results clearly indicate that the attachment to polystyrene is not the same as occurs in cell-cell adhesion during the mating reaction and that the binding of antibody to the cell surface may act to increase hydrophobic binding sites on the ciliary membrane. The finding that the Fab fragment prepared from purified immune IgG induces attachment as effectively as intact IgG clearly shows that crosslinking of cell surface components is not required for inducing attachment.